Age-related bone fracture is only partially explained by the reduction in bone mass that universally occurs with aging. The failure to predict fracture comes in part from a) the loss of geometrical information when projection x-ray methods are used to measure bone mass, b) ignorance of sufficiently detailed information about the failure properties of bone, and also c) ignorance of detailed in vivo loads on bones (Fracture risk is related to the ratio of load to strength, rather than to bone strength alone). A fourth barrier to accurately predicting fracture risk is that changes in bone tissue mechanical properties caused by age or disease related changes in the collagen, non-collagenous proteins or water content of bone are not detectable to xray methods. As a result of the invisibility of soft tissue changes to x-ray detection, there are age and menopause related changes in tissue material properties (in bone quality) that are invisible, not understood and that unpredictably increase bone fracture risk. Our data support the novel working hypotheses: 1) The viscoelastic properties of bone affect the fracture toughness and apparent strength of bone tissue, 2) The viscoelastic properties of ovine bone degrade after ovariectomy, causing a decrease in the strength and fracture toughness of bone tissue for rates of loading associated with falls. The main goal of the proposed work is to determine the onset time of viscoelastic property degradation and whether cortical bone can recover normal viscoelastic and strength properties with estrogen replacement.